An engine starting system having an engine starter has been known (for example, US 2004-0168664A1 corresponding to JP-A-2004-257369). In the system, an electric current is supplied to a starter motor of the engine starter to start an engine.
FIG. 4 is a schematic circuit diagram of an engine starting system 40 experimentally built to improve a conventional engine starting system. The system 40 includes two switches 101, 102, a starter relay 103 having a coil 103a and a relay switch 103b, a starter 104 having a starter motor 104a and a electromagnetic switch coil 104b for engaging the starter motor 104a to an engine, and an engine electronic control unit (ECU) 105. The switch 101 is turned on, when a key cylinder is turned to a start position. In an automatic transmission vehicle, the switch 102 is turned on when a gear is in a neutral position or a parking position. In a manual transmission vehicle, the switch 102 is turned on when a clutch pedal is pressed. When a driver turns the key cylinder to the start position as long as the switch 102 is ON, a current I 107 is supplied from a terminal 107 to the starter relay 103 and the engine ECU 105 receives a signal indicating that the key cylinder is turned on. Then, the engine ECU 105 provides a fuel injection signal to the engine and supplies a current I 105 to the starter relay 103. In this case, not only the current I 107 but also the current I 105 flow through the starter relay 103. Therefore, the starter relay 103 is surely activated during starting of the engine.
When the current I 105, or I 107 flows through the coil 103a of the starter relay 103 through the switch 102, an electromagnetic force is generated around the coil 103a. The starter switch 103b is attracted by the force and turned on. Consequently, a power source (battery) 106 supplies a current I 106 to the electromagnetic switch coil 104b and the starter motor 104. Thus, the engine is started.
The engine ECU 105 is supplied with electric power from the power source 106 through a terminal 108 and supplies the current I 105 to turn the starter switch 103b on. Likewise, the terminal 107 is supplied with electric power from the power source 106 and supplies the current I 106 to drive the starter motor 104a. The current I 106 becomes relatively large so that the power source voltage drops rapidly during starting of the engine.
Referring to a timing diagram of FIG. 5, the power source voltage drops below a predetermined reset level L2 at a time T1. The engine ECU 105 resets a supply of the current I 105 at the time T1, because the power source voltage is not high enough to operate the engine ECU 105 properly.
In this case, the current I 107 is continuously supplied from the terminal 107 to the coil 103a, as long as the key cylinder is in the start position, i.e., the switch 101 is ON. Therefore, the starter switch 103b stays ON and the starter motor 104a runs. As the starter motor 104a runs, a current load required to crank the engine decreases. The power source voltage increases accordingly between the time T1 and a time T2. Then, when the power source voltage reaches a predetermined return level L1 at the time T2, the reset state of the engine ECU 105 is released. Then, the engine ECU 5 restarts the supply of the current I 105 at a time T3 to start the engine. The engine can be thus started, even if the power source voltage drops rapidly during starting of the engine.
However, in a latest-type engine starting system in which an engine is started by pressing a push switch, there is no current path to supply the current I 107 when the engine ECU 105 falls into the reset state. Therefore, the engine cannot be started, if the engine ECU 105 resets the supply of the current I 105.
Specifically, the power source voltage returns to an initial level before starting of the engine, after the engine ECU 105 resets the supply of the current I 105 and the engine start-up sequence stops. Even when the engine start-up sequence is restarted and the engine ECU 105 restarts the supply of the current I 105, the power source voltage drops below the reset level L2 again. That is because the current load required to crank the engine has not been reduced. Therefore, no matter how many times the push switch is pressed for starting the engine, the engine cannot be started.